1. Field of the Invention
The present invention relates to a microscope optical system. More particularly, the present invention relates to an optical system which is able to compensate any unfavourable change of aberration as caused by the change of medium in the object space of an objective lens, especially a large aperture objective lens, for a microscope.
2. Description of Prior Art
An emulsion chamber is one of the known apparatus for recording the track of cosmic rays. The recording apparatus called emulsion chamber has a structure composed of alternating layers of lead plate and nuclear dry plate. The thickness of the lead plate is about 1 mm. FIG. 1 schematically shows the structure of a nuclear dry plate E as used in the emulsion chamber. As shown in FIG. 1, the nuclear dry plate is composed of a base 1 and two emulsion layers 2a and 2b between which the base 1 is sandwiched. As the base 1 there is commonly used methacrylic plate or polystrene plate. Examples of the combination of base plate and emulsion layers with respect to thickness are as follows:
(i) 0.08 mm thick emulsion layer+1 mm thick methacrylic plate+0.08 mm thick emulsion layer
(ii) 0.3 mm thick emulsion layer+0.15 mm thick polystyrene plate+0.3 mm thick emulsion layer
The track of cosmic ray is recorded on the emulsion layers of the nuclear dry plate. When the recorded track is examined through a microscope for three-dimensional coordinate measurement, it is necessary to observe not only the emulsion layer on the upper side of the base 1 as viewed from the microscope but also the underside emulsion layer through the base 1.
When the lens is then used in an oil immersion system of microscope objective lens in which the object space is filled with oil, the mediums existing in the object space such as oil, emulsion layer, base etc. have different refractive indexes and different values of dispersion. Therefore, the condition of aberration for microscopic observation of the nuclear dry plate at different depths is not constant but variable according to the change of depth ratio among the mediums, namely oil, emulsion layers and base in the object space. This change of aberration becomes unduly great in particular for a large numerical aperture (N.A.) objective lens. In the worst case, the microscopic observation becomes impossible. This will be further described with reference to MTF (Modulation Transfer Function) curves shown in FIG. 2.
By way of example, description is made in connection with an oil immersion type objective lens of focal length f=8.9 mm, magnification=50 X and N.A.=0.85.
Curve 3 in FIGS. 2A, B and C is an MTF curve obtained when the object space of the objective lens is filled with oil only. Curves 4 and 5 are MTF curves obtained when a part of the object space was replaced by the above described nuclear dry plate and the microscope was so set as to observe the further-most object point from the objective lens. The curve 4 is for the above shown combination (i) and the curve 5 for the combination (ii). FIG. 2A shows MTF at the center of the visual field (Ho=0). FIGS. 2B and C show MTF at a point spaced from the visual field center (Ho=-0.142) in meridional direction and sagital direction respectively (this description of MTF curves is applied also to the following drawings). In the shown example, the wavelength used is .lambda.=546.1 nm (e-ray). To this wavelength the mediums exhibit the following refractive indexes:
______________________________________ Oil n.sub.e = 1.51969 Emulsion layer n.sub.e = 1.53500 Methacylic plate n.sub.e = 1.49340 Polystyrene plate n.sub.e = 1.59765 ______________________________________
By inserting the nuclear dry plate, the MTF curve is shifted from 3 to 4 or 5. This unfavourable change of MTF is caused by the difference in refractive index between oil and nuclear dry plate.
Originally the object lens has been aberration compensated under the condition of the object space filled with oil. But, a part of the medium in the object space of the objective lens is replaced by another medium, combination (i) or combination (ii). The medium composed of the combination (i) has a lower refractive index than the original medium (oil) has. The medium composed of the combination (ii) has a higher refractive index than the original medium. Therefore, the state of aberration changes unfavourably in the direction toward undercompensated spherical aberration or in the direction toward overcompensated spherical aberration, whereby MTP is degraded as shown in FIG. 2.
Such unfavourable change of aberration may be compensated by changing the air gap within the objective lens as in the case of an objective lens for tissue culture microscope. Also, it may be compensated by moving any lens element of the objective lens. In either case, however, the focal length of the objective lens itself is changed by it and therefore the magnification thereof also changes. To carry out the coordinate measurement employing ITV camera (industrial television camera) or the like, therefore, it is required to continuously make a necessary adjustment every moment to cope with the magnification change. This correction is very complicated and is practically unrealizable. In addition, such compensating method by changing the air gap in the objective lens or by shifting any element of the lens necessarily involves the problem of eccentricity. Even when the position of the objective lens is completely adjusted to the height necessary for observation within the plane of the emulsion layer by moving the lens up and down precisely, the reference coordinate for microscopic examination will be lost due to the eccentric change.